Communication devices such as portable phones are being developed so as to work on multiple bands. In order to cope with multi-band working, one portable phone is equipped with RF devices that handles multiple RF signals such as filters, duplexers and amplifiers. It is desired to realize a compact module on which the RF devices are integrated in order to downsize the portable phones.
Such a compact module may use a multilayer substrate formed by stacking multiple insulation layers and multiple conductive layers. The characteristic impedance of a signal line included in a conductive layer depends on, for example the distance between related conductive layers and the dielectric constant of the insulation layer interposed between the conductive layers. For example, Japanese Patent Application Publication 2007-189152 (Document 1) discloses an art having a core of a dielectric, a ground layer that is provided between a signal layer and an outer layer and has an opening, and another ground layer that is provided in the conductive layer in which a signal layer is also provided. In this art, the characteristic impedance of the signal line depends on the distances between the outer layer and the ground layers. Japanese Patent Application Publication No. 2009-81423 (Document 2) discloses a multilayer substrate having a metal core having an opening in which an electronic component is accommodated.
In the art disclosed in Document 1, a signal interference may occur between the signal layer and the outer layer via the opening formed in the ground layer. In the art disclosed in Document 2, it may be difficult to realize both a thinned multilayer substrate and a desired characteristic impedance of the signal line.